In the area of security there is a need to provide materials that have non reproduceable effects and are preferably processable by printing on a substrate. Obvious effects include properties such as thermochromaticity and optically variable effects. Other features may be hidden such a the reflection of light with a certain polarisation state, infra-red reflecting or UV reflecting materials. These effects can only be seen with the use of viewing devices such as polarising films or specrophotometers.
Much work has focused on the use of cholesteric (or chiral nematic) liquid crystals (CLCs) as pigments. These pigments exhibit specific optical properties, such as angular colour dependence and reflection of circular polarised light, due to the chirality of the CLC material which induces a helically twisted molecular structure in the cholesteric phase. Suitable materials and methods of preparation of CLCs are described in prior art, for example in U.S. Pat No. 5,364,557 and U.S. Pat No. 5,599,412. Further descriptions of cholesteric pigment materials are given in GB 2 132 623, U.S. Pat No. 4,388,453 and WO 97/27251. The pigments described in these documents have three characteristic properties. Firstly, pigment particles are produced that reflect light within a specific range of wavelengths (dependent upon the composition of the LC material). Secondly the pigment particles have an angular dependent colour, i.e. the perceived colour changes as the viewing angle changes. Thirdly, the reflected light is circularly polarised, with the sense of polarisation being determined by the handedness of the chiral CLC material. However, CLC materials have several drawbacks. In general chiral materials for use in the CLC systems are difficult to prepare and expensive, especially if both chiral forms are required. Furthermore, for cholesteric materials an absorbing or black background is required to give best effects.
The use of nematic liquid crystal materials, which exhibit an untwisted liquid crystal phase, in birefringent films for security devices has also been reported in prior art. GB 2 357 061 describes a hot stamping foil for security applications, comprising a layer of polymerised or crosslinked nematic liquid crystal material with macroscopically uniform orientation applied onto a reflective layer. The birefringent nematic liquid crystal layer provides a hidden optical effect. The layer is invisible when viewed under unpolarised light, and produces a bright birefringence colour when viewed at between linear polarisers. The colour changes if the liquid crystal layer is rotated relatively to the polarisation direction of the polarisers.
However, the use of nematic materials as described in GB 2 357 061 has the following drawbacks. The materials need to be applied to a reflective background and subsequently cured using actinic radiation or transferred after polymerisation from a carrier film to a reflective film. In certain cases it may be necessary to pre-treat the substrate to ensure that the correct alignment of the materials is achieved. These steps are significantly different from standard printing processes and require additional machinery and techniques.
It was an aim of the present invention to provide new liquid crystal pigment particles, in particular for use in security devices, which do not have the disadvantages of prior art as discussed above. Another aim of the present invention was to provide methods of manufacturing the new liquid crystal pigments, which are especially suitable for mass production. Other aims of the present invention are immediately obvious to the expert from the following description.
It was found that the above aims can be achieved by providing solid liquid crystal flakes comprising a non-chiral liquid crystal material according to the present invention.
The flakes according to the present invention have several advantages compared to materials of prior art. For example, the use of chiral materials, which are difficult to prepare and expensive especially if both chiral forms are required, can be avoided. Furthermore, the nematic flakes of the present invention can produce a hidden image or hidden optical effect on a reflective background, whereas e.g. for cholesteric materials an absorbing or black background is required to give best effects.
Definition of Terms
The term ‘flakes’ as used in this application means small solid particles, preferably with dimensions of from 0.1 to 2000 μm, which are preferably platelet shaped and have average lateral dimensions that are several times larger than their thickness.
The term ‘vitrified material’ means a solidified glassy like, non-polymerised material.
The term ‘non-chiral’ material includes materials that consist exclusively of achiral compounds, as well as materials that consist of or comprise a racemate.
The term ‘film’ as used in this application includes self-supporting, i.e. free-standing, films or foils that show more or less pronounced mechanical stability and flexibility, as well as precoated, preprinted or laminated foils wherein the coating or printing can be partially or completely, as well as coatings or layers on a supporting substrate or between two or more substrates.
The term ‘marking’ includes films or coatings or layers covering the entire area of a substrate, as well as markings covering discrete regions of a substrate for example in the shape of a regular pattern or image.
The term ‘liquid crystal or mesogenic material’ or ‘liquid crystal or mesogenic compound’ should denote materials or compounds comprising one or more rod-shaped, board-shaped or disk-shaped mesogenic groups, i.e. groups with the ability to induce liquid crystal phase behavior. Liquid crystal compounds with rod-shaped or board-shaped groups are also known in the art as ‘calamitic’ liquid crystals. Liquid crystal compounds with a disk-shaped group are also known in the art as ‘discotic’ liquid crystals. The compounds or materials comprising mesogenic groups do not necessarily have to exhibit a liquid crystal phase themselves. It is also possible that they show liquid crystal phase behavior only in mixtures with other compounds, or when the mesogenic compounds or materials, or the mixtures thereof, are polymerized.
For the sake of simplicity, the term ‘liquid crystal material’ is used hereinafter for both liquid crystal materials and mesogenic materials, and the term ‘mesogen’ is used for the mesogenic groups of the material.
The term ‘director’ is known to the expert and means the preferred orientation direction of the long molecular axes (in case of calamitic compounds) or short molecular axis (in case of discotic compounds) of the mesogens in a liquid crystal material.
The term ‘planar structure’, ‘planar alignment’ or ‘planar orientation’ refers to a layer or film of liquid crystal material wherein the director is substantially parallel to the plane of the film or layer.
The term ‘homeotropic structure’, ‘homeotropic alignment’ or ‘homeotropic orientation’ refers to a layer or film of liquid crystal material wherein the director is substantially perpendicular to the film plane, i.e. substantially parallel to the film normal.
The term ‘tilted structure’, ‘tilted alignment’ or ‘tilted orientation’ refers to a layer or film of liquid crystal material wherein the director is tilted at an angle of between 0 and 90 degrees relative to the film plane.
The term ‘splayed structure’, ‘splayed alignment’ or ‘splayed orientation’ means a tilted orientation as defined above, wherein the tilt angle varies monotonuously in the range from 0 to 90°, preferably from a minimum to a maximum value, in a direction perpendicular to the film plane.
For sake of simplicity, a film comprising liquid crystal material with a planar, homeotropic, tilted or splayed orientation, alignment or structure is hereinafter also referred to as ‘planar film’, ‘homeotropic film’, ‘tilted film’ and ‘splayed film’, respectively.